Scrollvetica is a free app from my friend Jim Correia, which he wrote while using the developer seeds of Lion:

If you spend part of your time living in the future, with default
Magic Mouse and Magic Trackpad settings, you may find it difficult
to switch between the future and the present and maintain any sort
of input device sanity.

Scrollvetica is a simple hack which inverts all scrolling events
on Snow Leopard such that the effective scroll direction is in the
direction of finger movement.

My number one Lion tip: No matter how wrong it feels, stick with the new trackpad scrolling direction. Give it a week. At first it will drive you far crazier than you expect, but then you’ll get used to it.

I tested the Lion seeds on my secondary Mac, a then-brand-new-but-as-of-today-not-so-new 11-inch MacBook Air. The inverted trackpad scrolling drove me nuts. But after a week or so, it felt right. If you’re going to be using both Lion and Snow Leopard for now, running Scrollvetica on Snow Leopard will help you switch.